This invention relates to an ion sensor for various kinds of ion that can be employed in the examination of blood, etc. This invention also relates to a disposable simplified ion sensor plate partially constituting the ion sensor.
The ion sensor is generally formed of a structure comprising a silver/silver halide (Ag/AgX) electrode wherein a silver halide layer is deposited on a silver layer, an inner electrolyte solution, and an ion sensitive membrane (ISM). Since an inner electrolyte solution is employed in the ion sensor as described above, a vessel for accommodating the inner electrolyte solution is required. As a result, it has been very difficult to miniaturize the ion sensor or to make the ion sensor into a disposable type.
Under the circumstances, there has been developed a disposable simplified ion sensor plate for examining a blood sample, etc., and some kinds of it have been actually proposed.
As one example of such proposals, there has been known a differential type multi-ion sensor wherein a multi-ion sensor plate is connected with a measuring circuit thereby to measure concurrently the concentrations of many different kinds of ion. This multi-ion sensor is constructed as shown in FIG. 2. Namely, it comprises a multi-ion sensor plate main body 5 provided with 5 groups of electrode pairs, each group consisting of a test liquid-measuring electrode 2a and a reference electrode 2b, which are formed on a glass-epoxy resin substrate 1 by a process wherein copper electrodes are formed in advance by means of an etching process and then subjected to an electrolytic silver plating thereby to form a silver layer thereon, and with silver chloride layers 4a and 4b deposited on the silver layer formed on one end portion of each test liquid-measuring electrode 2a as well as on the silver layer formed on one end portion of each reference electrode 2b which faces the aforementioned one end portion of the test liquid-measuring electrode 2a, the other end portions of these electrodes 2a and 2b which are remote from the aforementioned one end portions being constituted as an outer electrode, respectively.
The multi-ion sensor further comprises a channel body 9 which is designed to be contactingly superimposed on the multi-ion sensor plate main body 5 and consists of a polyester film provided with through-holes 6a to 6e and 6'a to 6'e communicating with the silver chloride layers 4a and 4b of each group of an electrode pair, respectively, with an elongated hole constituting a liquid junction portion 7 which is formed to coincide with the forward ends of two channels of the upper cell to be explained hereinafter and is designed to allow a test liquid to be contacted with the reference liquid, and with through-holes 8a and 8b communicating respectively with the electrodes 2a and 2b of each group of an electrode pair.
Various kinds of ion-sensitive film-forming solution containing various kinds of ion-sensitive material such as a large cyclic compound called ionophore or an ion-exchange resin are individually applied through these through-holes 6a to 6e and 6'a to 6'e to the surfaces of silver chloride layers 4a and 4b, and then dried to form five kinds of different ion-sensitive films on the surfaces of silver chloride layers 4a and 4b.
The combined body consisting of this multi-ion sensor plate main body 5 and channel body 9 is then fitted in the recessed portions 10a and 10b of a lower cell 10 formed of a transparent acrylic resin plate. The resultant lower cell 10 is subsequently superimposed thereon with an upper cell 12 formed of a transparent acrylic resin via a pressure sensitive adhesive double coated tape so as to bond the lower cell 10 and the upper cell 12 to each other, thus packaging the aforementioned combined body.
The upper cell 12 is provided on the reverse surface thereof with a couple of channels 13 and 14 which are partitioned by a barrier wall and communicated with the through-holes 6a to 6e and 6'a to 6'e, respectively, each channel being formed of a U-shaped groove. A test liquid inlet portion 15 and a reference liquid inlet portion 16 are formed to communicate respectively with one end of the channels 13 and 14. A pair of air vent grooves 13a and 14a each being formed of a U-shaped groove of small width and bent into an L-shape are symmetrically disposed to communicate with the other end of the channels 13 and 14, respectively. The distal ends of these air vent grooves 13a and 14a are communicated with vertical passageways 13b and 14b formed at the forward end portions of the test liquid inlet portion 15 and the reference liquid inlet portion 16, respectively.
The upper cell 12 is further provided with terminal-insertion holes 18a and 18b for allowing the terminals of the measuring apparatus (not shown) to be inserted therein, the location of these terminal-insertion holes 18a and 18b coinciding with the exposed terminals of the outer electrodes of the test liquid-measuring electrode 2a and the reference electrode 2b.
When this multi-ion sensor plate is to be employed, a test liquid and a reference liquid are introduced by making use of a syringe, etc. into the test liquid inlet portion 15 and the reference liquid inlet portion 16, respectively. As a result, these liquids are supplied through the channels 13 and 14 to the ion-selective electrodes in the through-holes 6a to 6e and 6'a to 6'e, respectively, and at the same time, the test liquid is contacted with the reference liquid at the liquid junction portion 7. Under this condition, the terminals of measuring apparatus are inserted into the terminal-insertion holes 18a and 18b so as to be contacted with the outer electrodes respectively, thereby measuring the ionic components of the test liquid. As a result, the concentrations of 5 kinds of ion can be measured simultaneously with a single injection of these test liquid and reference liquid.
Even in the case where an ion-sensitive film is directly formed on the surface of Ag/AgX electrode as described above, the electric potential to be generated can be determined by the dissociation equilibrium represented by the formula: AgX.revreaction.Ag.sup.+ +X.sup.- as in the case of an the ordinary ion sensor. However, when a composition comprising polyvinyl chloride type resin and a plasticizer thereof is employed as an ion-sensitive film, the AgX constituting the underlying layer is dissolved mainly by the solubilizing power of this plasticizer, i.e. this ion-sensitive film is capable of functioning in the same manner as that of the inner electrolytic solution employed in the ordinary ion sensor.
In this case, the dissolving degree of the AgX layer can be controlled in a certain degree by the grain size and surface roughness of the AgX layer, and the electric potential to be generated can be controlled within a relatively limited range as made clear in our previous patent applications (Japanese Patent Applications H/1-135728 and H/1-222909).
By the way, in the structure where an ion sensitive membrane is directly formed on the surface of silver chloride layer without providing an inner aqueous electrolyte solution layer as mentioned above, the ion sensitive layer is formed by a process wherein a solution of a mixture containing a sensitive material or so-called ionophore which is selectively sensitive to an ion to be detected, and a salt such as an anion scavenger for eliminating the interference of anion originating from a test liquid or a reference liquid is prepared at first, and then coated on the surface of a matrix formed of polyvinyl chloride type resin for instance. However, in order to ensure the flexibility of this coated film so as to prevent the coated film from being peeled off from an underlying layer or from being cracked, a plasticizer is concurrently mixed into the ion sensitive membrane.
There is a possibility that plasticizer that has been incorporated into the ion sensitive membrane may be eluted into a test liquid or a reference liquid at the occasion of employing these liquids, or into a washing solution at the occasion of repeatedly using an ion sensor plate provided with this ion sensitive membrane. If the quantity of plasticizer thus eluted becomes large, the aforementioned ionophore and salt may also be eluted together with the elution of the plasticizer, thus deteriorating the property of the ion sensitive membrane. In view of this phenomenon, the mixing ratio of polyvinyl chloride type resin (the matrix of the ion sensitive membrane) and the plasticizer is generally confined to at most 35% by weight and 60-65% by weight, respectively based on the total weight of the ion sensitive membrane.
As mentioned above, since the dissolution degree of the AgX into the ion sensitive membrane changes not only due to a transient change in the dissolving process thereof into the ion sensitive membrane, but also due to changes in environmental conditions such as ambient temperature, ambient humidity, etc., if the AgX is left to dissolve spontaneously into the ion sensitive membrane, it is almost impossible to completely control these delicate changes in conditions even if it would have been possible to control the physical property of the AgX layer as described above. However, since there is a strong demand for a high precision ion sensor which is free from inconsistency of measured values, an ion sensor plate constituting an important component of the ion sensor is required to be subjected, after the manufacture thereof, to corrections in the initial characteristics thereof or to selection for picking up only those meeting the specification, as any discrepancy in measured values can be manifested as an error in generated potential of electrode in the inspection for quality control in the manufacturing site. Further, if any error in measurement is desired to be minimized, it is required to wait until an equilibrium state where the dissolution of AgX into the ion sensitive membrane can no more be proceeded any further is reached, thus inevitably raising the problem of cost increase in viewpoint of manufacturing steps.
The dissolution of AgX into the ion sensitive membrane can occur not only immediately after the manufacture of the ion sensor plate but also with the lapse of time. For example, even after 30 days, the electric potential at the interface between the AgX layer and the ion sensitive membrane may not be stabilized and the inconsitency of electric potential may become more conspicuous.
In the measurement of concentration of ions in blood, the effects of factors reflecting the quantity of hemocyte or protein in the blood, or so-called hematocrit (Hct) value (the volumetric ratio of erythrocyte in the blood) may be conceivable. However, no countermeasures have been taken as yet against this problem.
Properly speaking, it may be ideal to correct the measured value of ion concentration by taking this Hct value into consideration. However, since this Hct value varies depending on individual, i.e. the Hct value varies within the range of 42 to 45 in the case of adult men, and within the range of 38 to 42 in the case of adult women, and further, since there are various kinds of measuring methods, resulting in varied values depending on the method adopted, such a countermeasure has not been adopted because of poor reliability in measured values in general.
Under the circumstances, there is no other way but to adopt the values of ion concentration that can be obtained by making use of the aforementioned multi-ion sensor plate. However, there is a possibility of obtaining an abnormal value in ion concentration value depending on the hematocrit value (Hct %), i.e. depending on the quantity of hemocyte or protein in the blood. For example, when the Hct value is in the range of 40 to 50%, the value of Ca may become 0.1 to 0.2 mM (millimole/liter, the same hereinafter) lower or higher than the real value, and the value of Na may become about 2 to 5 mM lower or higher than the real value. In view of this problem, it is now desired to develop a method as well as an apparatus for measuring ion concentration which is free from the aforementioned undesirable effects.
In the method of forming an ion sensitive membrane by dripping a solution of raw material mixture in tetrahydrofran solvent onto an underlying layer, the surface or interior of the coated film is suffered from convection at the occasion of air drying after coating, thus turning the coated surface into the state of so-called orange peel and causing the coated film to lose its fluidity to be solidified as it is. As a result, it is impossible to assure the uniformity of the film.
Further, since polyvinyl chloride resin constituting the matrix cannot be sufficiently plasticized, it would become impossible to render the ion sensitive membrane to sufficiently wet to or spread over the underlying silver chloride layer, thus causing the contact surface thereof with the underlying layer to become insufficient.
Additionally, as being clarified in the invention recited in the appended claims, even if at least one kind of material for constituting the dissociation equilibrium of silver halide such as silver chloride or a material for dissociating the aforementioned at least one kind of material is added to the ion sensitive membrane in addition to the aforementioned components for the purpose of stabilizing the aforementioned dissociation equilibrium (AgX.revreaction.Ag.sup.+ +X.sup.-); or even if the same kind of ionophore as employed in the ion sensitive membrane is excessively added to the ion sensitive membrane; or even if an anion scavenger such as TPB (tetrakisphenyl borate) is excessively added to the ion sensitive membrane for the purpose of controlling the stability of the dissociation equilibrium of: Ag-TPB=Ag.sup.+ +TPB.sup.- substantially substituting the aforementioned dissociation equilibrium of: AgX.revreaction.Ag.sup.+ +X.sup.-, it is impossible to expect a sufficient effect of stabilizing these dissociation equilibriums, i.e. the electric potential to be generated at the interface between the silver halide layer of the electrode and the ion sensitive membrane.
If the composition of the ion sensitive membrane becomes non-uniform, and the contacting area of the ion sensitive membrane with the underlying layer becomes insufficient, the measured values of ion concentration to be detected from a test liquid becomes inconsistent, thus failing to obtain a satisfactory measurement precision.
In particular, if the electric potential measured of electrode of the main ion sensor body or ion sensor plate immediately after the manufacture thereof is inconsistent, i.e. if the initial characteristic of them is inconsistent, it is impossible to obtain a high measurement precision. As a result, it is required to correct the initial characteristics thereof or to select only those meeting the specification, thus inevitably raising the cost increase in viewpoint of productivity.
In the case where an ion sensor film is formed on an Ag/AgX electrode such as a silver/silver chloride electrode, it is required to stabilize the dissociation equilibrium of: AgX.revreaction.Ag.sup.+ +X.sup.- at the AgX layer. For the purpose of achieving this stabilization, a stabilizing agent may be incorporated into the ion sensitive membrane as suggested by this invention recited in the appended claims.
However, even when the resistance of the interface between the ion sensitive membrane and the AgX layer contacted with the ion sensitive membrane is high, the output of the multi-ion sensor is more likely to be influenced by the electric voltage to be generated by an electric resistance at the interface of electric current passing through a measuring circuit, and hence the precision of detected value obtained from the output, i.e. the measurement precision of ion concentration which is converted from the aforementioned detected value would be badly affected.
As measures for minimizing the electric resistance at the interface between the ion sensitive membrane and the AgX layer, the contacting area between the ion sensitive membrane and the AgX layer may be increased, or the electric resistance of the AgX layer may be minimized.
If the contacting area between the ion sensitive membrane and the AgX layer is desired to be increased, the particle diameter of the AgX may be reduced so as to increase the surface area of the AgX layer as is well known in the art. For example, Japanese Patent Publication H/7-214882 suggests to reduce the particle diameter of the AgX to 1 .mu.m or less. When an ion sensitive membrane containing polyvinyl chloride-based resin and a plasticizer for this resin is to be employed, the content of the plasticizer in the ion sensitive membrane may be increased so as to form an ion sensitive membrane having a sufficient pliability to conform with the fine rugged surface of the AgX layer, thereby increasing the contact area of the ion sensitive membrane. The effectiveness of this method is clarified in the invention to be recited in the appended claims.
However, the effects to be obtained by the measures to minimize the particle diameter of AgX or to increase the contacting area of the ion sensitive membrane as mentioned above are limited due to the limited area of the electrode on which the ion sensitive membrane is to be mounted. It is desired, in addition to these measures, to incorporate a potential-stabilizing agent into the ion sensitive membrane and at the same time, to modify the AgX layer per se so as to minimize the electric resistance of the AgX layer.